From the back of the book:
Once each month when the moon vanishes from the sky, dark horrors emerge from underground to resume their endless assault upon the surface world. Too often has the beleaguered town of Rookfast suffered this savage fate; now they must turn to a band of heroes for deliverance, or be destroyed by that which comes with the departing of the moon.
Fear the Moonless Night
Elemental Moon is the story of two artifacts that must be brought together to ensure the safety of a town in dire need. The adventurers must first survive the catastrophe of a raging forest fire, only to find themselves fighting for their lives in a city that’s been buried beneath the waves for a thousand years. Whether burned by fire or drowned by water, the heroes must find the mettle to recover a pair of sacred elements, and in so doing bring salvation to a town before the next disappearance of the moon.